1. Field of the Invention
The present invention relates to medical devices comprising bioactive coatings, and more particularly, to methods of sterilizing medical devices comprising bioactive coatings, while in their packaging, utilizing E beam sterilization techniques.
2. Description of the Related Art
Numerous metallic materials and polymeric materials have been utilized in the fabrication of implantable medical devices as well as coatings on implantable medical devices. Surface modifications and coatings of the same or different compositions are frequently used to further improve the biocompatibility and hemocompatibility of the implantable medical devices. The modification or coating of these devices typically requires several processing steps to complete. Substrates modified by each of these processes as well as the surface coatings require some manner of sterilization to ensure the sterility of the products for use in a patient. Currently utilized sterilization processes for bare metal devices may have potential drawbacks, for example, diminished coating stability, when utilized on coated devices, as the coating materials may not be compatible with these traditional sterilization methods.
Different methods of surface modification have been documented in the literature for the purpose of favorable host-material response. Several United States patents describe means and methods for coating medical devices, particularly those in contact with blood such as stents, but do not address the problem of subsequent sterilization (U.S. Pat. Nos. 4,656,083; 5,034,265; 5,132,108; 5,244,654; and 5,409,696). Palmaz et al., in a review of intravascular stents, are skeptical of the use of stent coatings (Palmaz, J., F. Rivera and C. Encamacion. Intravascular Stents, Adv. Vasc. Surg., 1993, 1:107-135). However, Kocsis et al. report that the use of heparin-coated stents was effective to reduce thrombogenicity of the stent surface (Kocsis, J., G. Llanos and E. Holmer. Heparin-Coated Stents, J. of Long-Term Effects of Medical Implants, 2000, 10 19-45).
Typical surface modifications include hydrophilic and/or hydrogel coatings such as polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), or Hyaluronic acid (HA), on the surface of cardiovascular implants, such as stents and pacemakers, or indwelling medical devices, topical wound healing applications, contact lenses, intraocular lenses, and the like. Hydrophobic or lubricious coatings are used for medical devices such as coronary or neurovascular guidewires, sutures, needles, catheters and trocars. Bio-active coatings are used for directed cell response such as cell adhesion molecules (CAM, such as RGD (amino acid sequence Arg-Glu-Asp), laminin, collagen, and the like) in tissue engineering applications or adhesion prevention coatings to be used on medical devices such as vena cava filters or small diameter vascular grafts. Coating materials also include infection resistance agents or antimicrobial agents. Some coatings also provide for sustained drug release such as sustained release of drug from stents, or as a hydrophobic overcoat to extend the release time of a drug loaded depot. Bio-active coatings containing therapeutic agents such as heparin, phosphoryl choline (PC), urokinase, and the like, are used for antithrombogenic properties.
The coatings may be used to deliver therapeutic and pharmaceutical agents including antiproliferative/antimitotic agents including natural products such as vinca alkaloids (i.e. vinblastine, vincristine, and vinorelbine), paclitaxel, epidipodophyllotoxins (i.e. etoposide, teniposide), antibiotics (dactinomycin (actinomycin D) daunorubicin, doxorubicin and idarubicin), anthracyclines, mitoxantrone, bleomycins, plicamycin (mithramycin) and mitomycin, enzymes (L-asparaginase which systemically metabolizes L-asparagine and deprives cells which don't have the capacity to synthesize their own asparagine; antiproliferative/antimitotic alkylating agents such as nitrogen mustards (mechlorethamine, cyclophosphamide and analogs, melphalan, chlorambucil), ethylenimines and methylmelamines (hexamethylmelamine and thiotepa), alkyl sulfonates-busulfan, nitrosoureas (carmustine (BCNU) and analogs, streptozocin), trazenes—dacarbazinine (DTIC); antiproliferative/antimitotic antimetabolites such as folic acid analogs (methotrexate), pyrimidine analogs (fluorouracil, floxuridine, and cytarabine), purine analogs and related inhibitors (mercaptopurine, thioguanine, pentostatin and 2-chlorodeoxyadenosine {cladribine}); platinum coordination complexes (cisplatin, carboplatin), procarbazine, hydroxyurea, mitotane, aminoglutethimide; hormones (i.e. estrogen); Anticoagulants (heparin, synthetic heparin salts and other inhibitors of thrombin); fibrinolytic agents (such as tissue plasminogen activator, streptokinase and urokinase), aspirin, dipyridamole, ticlopidine, clopidogrel, abciximab; antimigratory; antisecretory (breveldin); anti-inflammatory: such as adrenocortical steroids (cortisol, cortisone, fludrocortisone, prednisone, prednisolone, 6α-methylprednisolone, triamcinolone, betamethasone, and dexamethasone), non-steroidal agents (salicylic acid derivatives i.e. aspirin; para-aminophenol derivatives i.e. acetominophen; Indole and indene acetic acids (indomethacin, sulindac, and etodalac), heteroaryl acetic acids (tolmetin, diclofenac, and ketorolac), arylpropionic acids (ibuprofen and derivatives), anthranilic acids (mefenamic acid, and meclofenamic acid), enolic acids (piroxicam, tenoxicam, phenylbutazone, and oxyphenthatrazone), nabumetone, gold compounds (auranofin, aurothioglucose, gold sodium thiomalate); immunosuppressive: (cyclosporine, tacrolimus (FK-506), sirolimus (rapamycin), azathioprine, mycophenolate mofetil); Angiogenic: vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF); nitric oxide donors; anti-sense oligo nucleotides and combinations thereof.
Coatings may be formulated by mixing one or more therapeutic agents with the polymeric coating mixture. The therapeutic agent may be present as a liquid, a finely divided solid, or any other appropriate physical form. Optionally, the coating mixture may include one or more additives, for example, nontoxic auxiliary substances such as diluents, carriers, excipients, stabilizers or the like. Other suitable additives may be formulated with the polymer and pharmaceutically active agent or compound. For example, a hydrophilic polymer may be added to a biocompatible hydrophobic coating to modify the release profile, or a hydrophobic polymer may be added to a hydrophilic coating to modify the release profile. One example would be adding a hydrophilic polymer selected from the group consisting of polyethylene oxide (PO), PVP, PEG, carboxymethyl cellulose, and hydroxymethyl cellulose to a hydrophobic (co)polymer coating to modify the release profile. Appropriate relative amounts may be determined by monitoring the in vitro and/or in vivo release profiles for the therapeutic agents.
Methods for surface modification typically include a surface activation step followed by the coupling of the desired molecule. Surface activation is usually achieved by an energy assisted gas phase reaction (plasma, pulsed plasma, flow discharge reactive chemistry (FDRC), corona discharge, etc.) and/or activating the substrate with a highly reactive leaving group (N—OH succinimide, imidazole, etc.); Functionalization of the surface with self-assembly molecules (SAM, functional silanes and thiols); Controlled hydrolysis of the esters and amides at the surface (polyethylene terephthalate (PET), polylactic acid (PLA), polyglycolic acid (PGA), etc.). Coupling reactions are typically accomplished by carbodiimide chemistry, reductive amination, malemide-thiol reactions, etc.
Photochemical surface modifications are usually preferred since this method typically does not require a prior surface activation step. Arylketone based chemistry, azide chemistry, acrylate chemistry are key examples.
Regardless of the type of coating, sterilization of the final product, as stated above, may cause potential problems. Conventional sterilization methods such as hot steam, radiation (gamma and E beam), and ethylene oxide may negatively impact the activity of the coating. For example, medical devices are normally sterilized by a terminal sterilization process such as ethylene oxide (EtO) sterilization, gamma sterilization, or more recently E beam sterilization. EtO sterilization is mild toward metal and polymer based medical products such as catheters, bare metal stents and early generation drug eluting stents. It is a long and often cumbersome process that needs fine tuning of process parameters such as duration, temperature, humidity, the ratio between carrier gas and moisture, and extensive degassing processes to remove the residual EtO after the process, More importantly, the very mechanism by which EtO kills pathogens (the disruption of the nucleic acids) in the presence of moisture may also be detrimental to sensitive chemical compounds and most biological molecules. Proteins, peptides and gene products are most prone to destruction of EtO. Gamma sterilization which does not involve moisture is extremely energy intensive to be useful in sterilizing most biological containing devices and drug device combination products. E beam, sterilization, which is electrically generated gamma radiation, is also energy intensive and is also known to be potentially destructive to many biologics.
Ethylene oxide sterilization (EtO mixed with water vapor) has been known to decrease the activities of biologically active surfaces such as heparin coated surfaces. In addition, the presence of water vapor in the EtO process is also known to have a negative impact the shelf life of sterile medical devices containing heparin surfaces. Other energy intensive processes such as gamma and E beam sterilization processes have been shown to cause a decrease in the activity of bioactive coatings, see for example U.S. Pat. No. 6,787,179.
Experience with heparin coated stents and EtO sterilization has shown that in this type of sterilization process, it is difficult to control and may cause a significant decrease in heparin activity. EtO may also cause fluctuations of the heparin activities from one manufacturing site to another. Small changes of EtO sterilization conditions could lead to a wide variation of heparin activity and consequently the release specification and shelf life of heparin coated stents.
In the era of the drug eluting stents, heparin and other bioactive coatings or surfaces will be in close vicinity of a drug such as sirolimus and the drug carrier such as PLGA polymers. In addition to the sensitivity of heparin towards EtO, both sirolimus and biodegradable polymers are known to retain substantial amounts of EtO post-processing. It is therefore advantageous to use alternative methods such as E beam to terminally sterilize heparin coated drug eluting devices. In the literature, it is generally not advised to use high energy process such as E beam to sterilize a biologics containing pharmaceutical and/or drug device combination products. Instead, expensive aseptic manufacturing, filtration/lyophilization processes are commonly used to ensure sterility of the final packaged products.
Given the above limitations of conventional sterilizations including ethylene oxide, E beam and gamma radiation processes, they have not been routinely used to sterilize medical devices that contain a biologically active component such as a heparin coating. Accordingly, there exists a need for a convenient terminal sterilization process that ensures both the sterility of a medical device and the activities of its biological coating.